Transient solid-phase crystallization kinetics of undoped silicon films prepared by chemical vapor deposition of silane has been studied with in situ x-ray diffraction. The experiments were performed under vacuum in a high-temperature chamber mounted on the circle of a step-by-step goniometer. By studying the Bragg-peak-intensity dependence on the isothermal-annealing time (t) over a wide temperature range 594°CT658°C, we have directly determined the volume of the crystallized phase and the grain size. We report direct experimental evidence that solid-phase crystallization occurs first at the substrate-layer interface through the growth of three-dimensional islands after a fast transient heterogeneous nucleation mechanism. We observe a relative crystal-lattice expansion of about 3×10-3 at the beginning of this regime. The decrease of the corresponding elastic-energy contribution to the free energy of the system with increasing annealing time enhances the driving force of the amorphous-crystalline phase transition. Our experimental data give indirect evidence of the presence of strained submicrocrystallites in the amorphous phase as a precursor state of the crystalline state. According to the models generally used, a phenomenological sequence describing the solid-phase crystallization kinetics is also given. © 1989 The American Physical Society.